1. Technical Field
This invention relates to stepping motors, and more particularly to an improved arrangement of the windings on the stators of such motors.
2. Background Art
Stepping motors are well-known devices for providing incremental motion and have utility in a wide variety of applications requiring the accurate positioning of mechanical and electromechanical machinery. Typically, a stepping motor includes a stationary member, or stator, and a moving member, the two members having magnetically interacting structures, with successive changes in electromagnetic fields causing incremental changes in the position of the moving member. The stator typically includes a plurality of electromagnetic salient poles. Each pole is excited by electric current passing through a coil wound upon the pole. The coil on one pole may be connected in series or parallel with the coil(s) of one or more additional poles in such a sense that the flux polarities of the poles are appropriate. Such a coil or connected coils are commonly referred to as a "winding".
Heretofore, such a stepping motor winding has typically been "bifilar" in arrangement; that is, the winding consisted of two conductors wound physically parallel. Originally, the bifilar winding was driven with a unipolar drive, whereby one of the parallel conductors was energized with electric current flowing in one direction to provide one polarity of the pole on which the bifilar winding was wound and, when the opposite polarity was required, the other parallel conductor was energized with electric current flowing in the opposite direction, producing the opposite polarity of the pole.
In a further development, bipolar drives were applied to excite the windings of a bifilar-wound stepping motor. This provides simpler drive circuitry and also permits either series or parallel connection of the two conductors, thus providing high impedance for optimum performance at low stepping speeds (series connection) and low impedance for optimum performance at high stepping speeds (parallel connection). In the series connection arrangement, the end of one parallel conductor is connected to the beginning of the other parallel conductor, so that the exciting current flows in one direction through the first conductor and then in the same direction through the second conductor. In the parallel connection arrangement, the end-to-beginning connection is open and current is connected to flow in parallel through both conductors from their beginnings to their ends.
While the bipolar drive has heretofore been found generally satisfactory for exciting bifilar windings, the arrangement has been found to have certain disadvantages when the conductors are connected in series. In order to improve stepping performance, increasingly higher drive voltages are used across the stepping motor windings, but these voltages are limited by the high potential which necessarily results between the individual conductors of the bifilar winding when connected in series. This high potential can cause breakdown of the insulation covering (and separating) the conductors. Another disadvantage of the bifilar winding connected in series is that the drive system must deal with rather large voltage "spikes" due to inductive coupling between the two conductors, which also can cause insulation breakdown.